Field electron emission materials and devices

ABSTRACT

A field electron emission material has a substrate with an electrically conductive surface. Electron emission sites on the conductive surface each include a layer of electrically insulating material to define a primary interface region between the conductive surface and the insulating layer, and a secondary interface region between the insulating layer and the vacuum environment,. Each primary interface region is treated or created so as to enhance the probability of electron injection form the conductive surface into the insulating layer. Each primary interface region after such treatment or creation is either an insulator or graded from conducting adjacent the conductive surface to insulating adjacent the insulating layer.

This invention relates to field electron emission materials, and devicesusing such materials.

There have been many proposals for broad-area field electron missionmaterials, many or most of which concentrate on the use of diamond oramorphous carbon as an emitting material of special significance. In thecontext of this definition, a broad-area field emitter is any materialthat by virtue of its composition, micro-structure, work function orother property emits useable electronic currents at macroscopicelectrical fields that might be reasonably generated at a planar ornear-planar surface.

The reader is referred to UK Patent 2 304 989 (Tuck, Taylor & Latham)for examples of emitting materials, including many other than diamond.The present application relates particularly to field electron emissionmaterials involving a primary interface region between a conductivesurface, or an electrically conductive particle on it, and an insulatinglayer, and a secondary interface region between that insulating layerand the environment in which the field electron emission material isdisposed.

A critical issue in insulator-based field emitting systems is theinjection of electrons from a substrate (often a metal) into theconduction band of the insulator.

FIG. 1a is a reasonable representation of the current state of knowledgeof such systems, although this still falls short of an exactdescription. In particular the sharp cut off in the density of states atthe band edges is unlikely in highly heterogeneous amorphous materials.However, with these caveats in mind, such a diagram is a usefulrepresentation. Electron emission through a dielectric coating iseffectively controlled by three factors: injection of the electrons 1503into the dielectric from the conducting substrate 1500; transportthrough the dielectric to the surface as indicated by line 1511; andsubsequent escape through or over the surface barrier 1506 into thevacuum 1502. A practical insulating layer will have both donor 1507 andacceptor defect sites 1509 in the band gap. The most notable effect iswhen there are donor states in the band gap relatively close to thebottom of the conduction band. In this case electrons from the donorstates 1507 tunnel back into the metal and a Schottky barrier 1510 isformed, see also FIG. 1(b), which enables electrons to tunnel through itfrom the metal into the conduction band. Bayliss and Latham (K. H.Bayliss and R. V. Latham, Proc. Roy. Soc. Lond. A 403 (1986) 285-311)have described the conditions required for forming such a Schottkybarrier and its significance to electron emission into the dielectric.The Schottky barrier has an associated forward voltage drop. Thisbecomes a particular issue as the particle size is reduced in themetal-insulator-metal-insulator-vacuum (MIMIV) emitters described byTuck, Taylor and Latham (UK Patent 2304989) to enable them to be used ingated structures such as those described in our patent application GB 2330 687. Whilst the electric field across the MIM region of a MIMIVemitter can be maintained by reducing the insulator thickness, theabsolute voltage will fall to values below the forward voltage drop ofthe Schottky barrier thus stopping injection of electrons into theinsulator.

A more general discussion of the metal-insulator contact in the case ofdiamond and diamond-like carbon is given by Robertson (J. Robertson,Mat. Res. Soc. Symp. Proc. 471 (1997) 217-229).

Transport through the dielectric depends critically on its nature. Forrelatively defect-free material, transport will be in the conductionband, with lattice scattering limiting conduction. Electrons may becomeballistic rather than staying close to the bottom of the conduction band(D. J. DiMaria and M. V. Fischetti, Excess electrons in dielectricmedia, eds Ferradini and Jay-Gerin, p315-348, (CRC Princetoun:1991) ISBN0849369622). By contrast, in a glassy material, with many donor andtrapping sites, conduction will be dominated by the Poole-Frenkeleffect, field-assisted ionisation of donors and traps, and the electronswill remain close to the Fermi level. In general conduction is non-ohmicwith evidence of saturation effects, presumably due to space charge insome cases.

The final step is the emission of electrons from the dielectric surfaceinto vacuum. In the case of hydrogen terminated diamond which has anegative electron affinity, and with the electron transport in theconduction band, there is no barrier to overcome and all electronsarriving at the surface will be emitted. In the case of a low positiveelectron affinity, such as an un-terminated diamond surface, there isusually sufficient electron heating in the transport to the surface toallow emission through thermionic and thermally enhanced tunnelling. Forhigher electron affinities, either the field at the surface must be highenough to enable tunnelling or there must be sufficient ballisticelectrons that can pass over the barrier. Otherwise the surface must bemodified to lower the effective electron affinity. Two possible means ofachieving this lowering of the surface barrier are either modifying thesurface composition e.g. by caesiating the surface or emptying surfacedonor states to leave a positively charged surface. The latter is thebasis of the forming mechanism proposed by Bayliss and Latham.

An emitter of this type has initially to undergo a forming process. Arelatively high switch-on field has to be applied to the device toobtain emission, but after removing this field, a much lower thresholdfield is required for emission. The actual mechanisms responsible forthis behaviour are very difficult to establish because of the smalldimensions of the conducting channels. Dearnaley et al. (G. Dearnaley,A. M. Stoneham and D. V. Morgan, Rep. Prog. Phys., 33, (1970) 1129-1191)suggest the formation of conducting filaments in the films for MIM(metal-insulator-metal) structures, while Bayliss and Latham suggestthat a positive space charge is established in the insulator and at itssurface.

Many papers on diamond and diamond-like-carbon field emitters make nomention of any forming process. However, a forming process is describedfor diamond emitters both by Xu et al. (N. S. Xu, Y. Tzeng, and R. V.Latham, J. Phys. D 26 (1993) 1776-1780) and by Givargizov et al. (E. I.Givargizov, V. V. Zhirnov, A. V. Kuznetsov and P. S. Plekhanov, J. Vac.Sci. Technol, B 14 (1996) 2030-31). It seems probable that other workersin this area concentrate on the reversible I-V characteristics of theemitters and may overlook the initial forming process.

It is probable that no one mechanism is appropriate to all situationsand that a combination may apply in many cases.

For diamond films, the limiting factor to emission has been found bymany workers to be the metal-diamond back contact (e.g. M. W. Geis, J.C. Twichell and T. M. Lyszczarz, J. Vac. Sci. Technol. B 14, (1996)2060-67) and U.S. Pat. No. 5,713,775. However, no systematic method ofovercoming this problem has been described.

Examples of ad hoc solutions are as follows.

Geis et al. showed that emission thresholds could be greatly reduced byintroducing nitrogen into the diamond. The nitrogen defects are closeenough to the conduction band to allow a Schottky barrier to be formed,reducing the field necessary to inject electrons into the diamondconduction band. Geis et al. considered also that “roughening” of thesurfaces between metal and diamond was of considerable importance,roughening being of the order of 10 nm.

In fact it is likely that many examples of diamond and carbon-basedfilms have an interface roughness of this order without intentionaltreatments. What is really needed is a more general strategy that can beapplied to interfaces whether they are rough or smooth.

Schlesser et al reported improved emission for an annealedmolybdenum-diamond interface (R. Schlesser, M. T. McClure, W. B. Choi,J. J. Hren and Z. Sitar, Appl. Phys Lett. 70 (1997) 1596-98)

Chuang et al reported improved emission for diamond deposited onto anannealed gold layer on silicon (F. Y. Chuang, C. Y. Sun, H. F. Cheng andI. N. Lin, Appl. Phys. Lett. 70 (1997) 2111-3).

In the last two cases it is probable that the Schottky barrier has beenreduced or eliminated through the formation of some form of an ohmiccontact. It is however difficult to be certain of the operatingmechanisms of the recipes described in these publications asinsufficient information is given about the nature of the diamond films.

Two more brief and general disclosures of emission from diamond filmsare C. Kimura, K. Kuriyama, S. Koizumi, M. Kamo and T. Sagino, PaperL-2, and T. Yamada, A. Sawabe, K. Okano, S. Koizumi and J. Itoh, PaperP-45, both papers being from IVESC '98—The International Vacuum ElectronSources Conference held in Tskuba City, Japan. The first of these papersdiscusses the use of titanium and gold with phosphorus-doped diamondfilms, and notes the effect of different resistivities of the diamondfilm. The second of these papers discusses the use of both titanium andgold with nitrogen-doped and boron-doped diamond emitters. Both papersemphasise the perceived importance of diamond as a choice of emittermaterial to achieve good emission characteristics, but disclose nogeneral teaching as to how to achieve good emission characteristics frommaterials generally.

Preferred embodiments of this invention aim to provide a systematicmethod for producing optimised low manufacturing cost field emittermaterials based upon insulating coatings that have both a low emissionthreshold field and a controlled saturation above a chosen currentdensity.

According to one aspect of the present invention, there is provided amethod of creating a field electron emission material, comprising thesteps of:

providing a substrate having an electrically conductive surface;

providing a plurality of electron emission sites on said conductivesurface, each of said sites including a respective layer of electricallyinsulating material to define a primary interface region between saidconductive surface, or an electrically conductive particle on it, andsaid insulating layer, and a secondary interface region between saidinsulating layer and the environment in which the field electronemission material is disposed; and

treating or creating the primary interface region of each said layer soas to enhance the probability of electron injection from said conductivesurface into said layer, such treatment or creation comprising:

depositing a layer of material between said conductive surface andinsulating layer, which layer of material has properties intermediatethose of said conductive surface and said insulating layer; or

doping said conductive surface and/or insulating layer with a materialthat segregates out at said primary interface region during subsequentprocessing; or

reaction of the materials of said conductive surface and insulatinglayer; or

creating said primary interface region as a region of high electricallyactive doping, high defect density or intermediate chemical composition:

such that said primary interface region after said treatment or creationis either an insulator or graded from conducting adjacent saidconductive surface to insulating adjacent said insulating layer.

Said layer of material between said conductive surface and insulatinglayer may be created by a gradual change in stoichiometry, compositionor doping of the material of the layer, to reduce discontinuity.

A method as above may further comprise the step of selecting theproperties of said insulating layer of each said site between itsrespective primary and secondary interface regions to limit the emissioncurrent flowing through said layer to a predetermined value.

Preferably, said primary interface region is a layer of material of lowwork function.

Preferably, said primary interface region is created as a region of highdoping, defect density or intermediate composition.

Such a region of high defect density may be created by heat treating amajor portion of a highly defective insulator material to create saidinsulating layer, whilst avoiding heat treatment of an end portion ofsaid highly defective insulator material, which end portion then remainsas said region of high defect density.

Preferably, said secondary interface region is provided by modifying thesurface of said insulating layer, to enhance the probability of electrontransmission from said insulating layer to said environment.

Modification of said surface may be by a local increase in defectdensity of the material of the insulating layer.

Modification of said surface may be by a gradual change instoichiometry, composition or doping to reduce discontinuity.

Modification of said surface may be by local heat treatment of saidinsulating layer.

Said electron emission sites may be defined by tips or projectionscreated on said conductive surface.

Said electron emission sites may be defined by electrically conductiveparticles coated on said conductive surface.

Said secondary interface region may be defined at a region of saidinsulating layer between a respective said particle and said conductivesurface.

Said secondary interface region may be defined at a region of saidinsulating layer which is provided on a portion of a respective saidparticle which faces away from said conductive surface.

Each said particle may have a first layer of electrically insulatingmaterial between said substrate and particle and a second layer ofelectrically insulating material between said particle and environment,the arrangement being such that, in use, electron emission takes placeby injection of electrons through one said primary interface regiondefined between said substrate and said first insulating layer, byinjection of electrons through another said primary interface regiondefined between said particle and said second insulating layer, and bytransmission of electrons through said secondary interface regiondefined between said second insulating layer and said environment.

Preferably, said first and second insulating layers are provided byrespective portions of a common electrically insulating material.

Said insulating layer may be of a material other than diamond.

Preferably, the distribution of said sites over the field electronemission material is random.

Said sites may be distributed over the field electron emission materialat an average density of at least 10² cm⁻².

Said sites may be distributed over the field electron emission materialat an average density of at least 10³ cm⁻², 10⁴ cm⁻² or 10⁵ cm⁻².

Preferably, the distribution of said sites over the field electronemission material is substantially uniform.

The distribution of said sites over the field electron emission materialmay have a uniformity such that the density of said sites in anycircular area of 1 mm diameter does not vary by more than 20% from theaverage density of distribution of sites for all of the field electronemission material.

Preferably, the distribution of said sites over the field electronmission material when using a circular measurement area of 1 mm indiameter is substantially Binomial or Poisson.

The distribution of said sites over the field electron emission materialmay have a uniformity such that there is at least a 50% probability ofat least one emitting site being located in any circular area of 4 μmdiameter.

The distribution of said sites over the field electron emission materialmay have a uniformity such that there is at least a 50% probability ofat least one emitting site being located in any circular area of 10 μmdiameter.

The invention extends to a field electron emission material produced byany of the above methods.

According to a further aspect of the present invention, there isprovided a field electron emission device comprising a field electronemission material as above, and means for subjecting said material to anelectric field in order to cause said material to emit electrons.

It will be appreciated that the electrical terms “conducting” and“insulating” can be relative, depending upon the basis of theirmeasurement. Semiconductors have useful conducting properties and,indeed, may be used in the present invention as said conductive surfaceor particles. In the context of this specification, the or each saidconductive surface or particle has an electrical conductivity at least10² times (and preferably at least 10³ or 10⁴ times) that of saidelectrically insulating material.

For a better understanding of the invention, and to show how embodimentsof the same may be carried into effect, reference will now be made, byway of example, to the accompanying diagrammatic drawings, in which:

FIG. 1a shows the band structure for an insulator in contact with ametal under conditions of high electric field;

FIG. 1b shows the band structure for an insulator in contact with ametal with a matching layer of high doping level or intermediatecomposition under conditions of high electric field;

FIGS. 2a to 2 i show various optimised insulating coatings for fieldemission;

FIGS. 3a to 3 d show applications of optimised contacts between metalsand insulators in field emitter materials and devices; and

FIGS. 4a to 4 d show applications of optimised insulator surface layersin field emitter materials and devices.

Preferred embodiments of the invention aim to improve the performance ofemitters based upon low cost materials and deposition systems, althoughthe teachings of this work are equally applicable to diamond and carbonbased emitters.

The first essential is to have as low a barrier as practicable for theinjection of electrons into the dielectric. This requirement implieseither minimising the width of the Schottky barrier or forming a trulyohmic contact.

The createation and control of metal-semiconductor interfaces is wellestablished in that art, see for instance E. H. Rhoderick and R. H.Williams, Metal-semiconductor contacts, Clarendon Press, Oxford, 1988.It is known that for semiconductors a low Schottky barrier or an ohmiccontact may in principle be obtained by a careful selection of thecontact materials. However, the vast majority of contacts insemiconductors depend on heavily doping the semiconductor in theinterface region to make the depletion layer at the interface very thin.Bayliss and Latham show that a population of impurity and donor levelsat a concentration of about 10⁹ cm⁻³ near the bottom of the conductionband is necessary to form the type of Schottky barrier required toexplain pre-breakdown emission from MIV sites on cathode surfaces.Increasing the defect population above 10¹⁹ cm⁻³ will allow a furthernarrowing of the depletion layer.

To be a useful emitter in field emission devices, the bulk of thedielectric must be sufficiently insulating at the device operatingtemperature to maintain any space charge created in the forming processbut pass the full operating current for the device at an external fieldof ^(˜)10 MV m⁻¹ V/micron). The conductivity and any tendency to spacecharge limitation may be controlled both by limiting the donor and trapdensities and by the thickness of the coating. The optimum densitieswill be lower than those required at the metal-insulator interface toreduce the thickness of the Schottky barrier. In a practicallyrealisable system the donor and trap densities will most easily be aproperty of the bulk insulator composition and deposition method, andconsequently, for optimum performance, modification of the interfacebetween the insulator and metal is required.

Alternatively, the outer regions of a highly defective insulator may belocally heat-treated, as by annealing, for example, with a laser, tocreate the desired structures.

To enable the reader to better understand the preferred embodiments ofthe inventions described herein, the electronic situation in a MIVstructure without modification of the metal-insulator contact will bedescribed with reference to FIG. 1a. The figure depicts a metallicsubstrate 1500, an insulator layer 1501 and a vacuum region 1502. Theupper edge of the valence band 1504 and conduction band edge 1505 areshown. In the steady state following forming (see Bayliss and Latham)electrons 1503 tunnel into the insulator and are transported in thepenetrating field by Poole-Frenkel hopping between the donor 1507 andacceptor 1509 states. Vacancies 1508 in the donor levels create a spacecharge which maintains the conducting channel once the external fieldhas been removed. Electrons are heated in the penetrating field and maytunnel through or be emitted over the field-modified surface potentialbarrier 1506.

Again with reference to FIG. 1a, control of the donor and trap densitiesin the near surface region 1512 is beneficial to emission. By the nearsurface region we mean the area ^(˜)10 nm below the surface. Since theforming mechanism is initiated by tunnelling of electrons from thesurface and near surface donors, a modest increase in the concentrationof these donors will allow the switch-on field to be reduced.

In another preferred embodiment of the present invention there isprovided, with reference to FIG. 1b (wherein the symbols for donors,acceptors and ionized donors are the same as in FIG. 1a) an insulatinglayer 1546 the composition of which (with respect to density of chargecarriers, mobility, trap density et cetera) is chosen such that ifrequired, once electroforming has taken place, current limitation occursat the desired value. There is then created a layer of high doping,defect density or intermediate composition 1540 disposed between thesubstrate and insulator layer. Said layer reduces the thickness of thedepletion region 1541 of the Schottky barrier thus facilitating thetunnelling of electrons into the insulator 1546. A magnified view of thedepletion region is show as 1544 with the symbols having the samemeaning as those in Figure la. Said layer may either be:

deposited on the metal substrate prior to coating with the insulator;

created in situ by doping the substrate or insulator with material thatsegregates out at the interface during subsequent processing;

or created by choosing a substrate and an insulator such that they reacttogether to create said layer.

In another preferred embodiment of the present invention there isprovided an emitter layer wherein the surface of the insulator presentedto the medium into which the electrons are emitted (often a vacuum) ismodified to facilitate electron emission. Said modifications mayinclude:

a local increase in defect density relative to the bulk of theinsulating layer;

a gradual change in stoichiometry, composition or doping relative to thebulk of the insulating layer, thus avoiding a discontinuity.

Embodiments of this invention may have many applications and some willbe described by way of the following examples. It should be understoodthat the following descriptions are only illustrative of certainembodiments of the invention. Various alternatives and modifications candevised by those skilled in the art.

Field emission from a clean metal surface takes place at electric fields^(˜)1000 MV m⁻¹ Consequently, an arrangement with a beta factor greaterthan unity is required. This is usually a fabricated atomically sharppoint. By beta factor we mean the enhancement of the macroscopic fieldby the pointed structure. Coating the surface with an insulator layer,especially an optimised one as described herein, and then forming aconducting channel reduces the required field by approximately one orderof magnitude. Given that safe electrical fields within vacuum electronicdevices are approximately 10 M V m⁻¹, structures with beta factors of^(˜)10 are required for a technologically useful field emissionmaterial. Beta factors of this magnitude can be realised by relativelyblunt microfabricated tips with radii of curvature of 20 nm to 100 nm orrough surfaced particles.

FIGS. 2a to 2 j show conducting surfaces 1600 with beta factors of^(˜)10 coated with various layers.

EXAMPLE 1

Moving now to FIG. 2a, a conducting layer 1601 comprises a gold-titaniumalloy, the titanium concentration being a few atomic percent. Such alayer may be deposited by sputter coating from a target with therequired alloy composition. An insulator layer 1602 is composed ofsilica which may be, by way of example, deposited by sputter coating,plasma deposition or by heating a layer of polysiloxane spin-on glass to^(˜)500° C. Upon heating, the titanium will segregate out of thegold-titanium layer and concentrate at the interface with the silica.Titanium will reduce silica to silicon. As a result a region 1603 shownin FIG. 2b will be created, having properties intermediate those of theconducting and insulating layers. Thus, this will be graded fromgold/titanium through titanium, silicon, the sub-oxides of silicon tosilica. Said graded layer will reduce the width of the Schottky barrierand facilitate the injection of electrons into the insulator. Similarresults may be obtained with gold-hafnium, gold-zirconium alloys andalloys containing glass forming elements such as boron, silicon,vanadium, phosphorous, selenium, tellurium, arsenic and antimony.

EXAMPLE 2

Moving now to FIG. 2c, a layer of chemically reactive (often reducing)material 1605 is deposited on an optional additional conducting layer1606 by means of spin coating, electrophoresis or other method. Thelayer 1605 reacts with either or both of the insulating layer 1602 andthe conducting layer 1606 (or substrate 1600) to produce theintermediate layer 1607 shown in FIG. 2d. A suitable material for layer1605 is colloidal graphite which, because of its high surface energy,can, following heat treatment, reduce silica, a likely material for theinsulator, to silicon sub-oxides. This produces a layer of intermediateproperties that facilitates the tunnelling of electrons from thesubstrate into the insulator.

EXAMPLE 4

Moving now to FIG. 2e, the substrate 1600 is coated with a layer ofresinate gold ink 1610 by, for example, spraying, screen printing,brushing or spin coating. Such resinate golds are well known in thedecorative glass and pottery industries and to a lesser extent forelectronic applications e.g. Koroda U.S. Pat. No. 4,098,939. Someaspects of their chemistry are described by A A Milgran (Migram, A. A.journal Electrochemical Society, Solid State Science. February. 1971,pp287-293). Milgram states that the two principle ingredients inaddition to the gold chemicals are rhodium, which controls grain growthto produce a continuous film, and chromium which aids adhesion to thesubstrate.

On firing said resinate gold ink layer in air, a continuous gold film(FIG. 2f) 1611 ^(˜)100 nm thick doped with rhodium and chromium isproduced.

Moving to FIG. 2g, a layer of insulator 1612 such as silica or glass isnow deposited by physical or chemical means—a number of such methodshaving been described previously. Heating of the completed layeredstructure causes a reaction at the interface between the additives inthe gold layer 1611 and the insulator 1612 to produce a graded structure1613 comprising, it is believed, a network of silicates and chromates.This produces a layer of intermediate properties that facilitates thetunnelling of electrons from the substrate into the insulator.

EXAMPLE 4

Moving now to FIG. 2h, the substrate 1600 is coated with a SiO_(x) layerin a plasma enhanced CVD (PECVD) reactor using a silane and oxygen mix.Initially the gas mixture is adjusted to deposit a layer 1622 which isstoichiometrically close to SiO. After ^(˜)10 nm of the layer has beendeposited the gas mixture is changed to move the stoichiometry of thelayer 1621 closer to SiO₂.

Alternatively the properties may be changed by varying a dopant such ascarbon added by bleeding in an appropriate gas (e.g. methane.) to thesilane-oxygen mixture.

Either approach produces a layer of intermediate properties thatfacilitates the tunnelling of electrons from the substrate into theinsulator.

EXAMPLE 5

Moving now to FIG. 2i, layers 1631 and 1632 are the same composition asthose in Example 5 (FIG. 2i). However, in this case the gas mixture ischanged towards the end of the deposition process to increment thestoichiometry of the surface region 1633 away from SiO₂ towards, but notapproaching, SiO. The thickness of layers 1631 and 1633 is of the orderof 10 nm. This modifies the surface in a way that facilitates electronemission.

EXAMPLE 6

The metal surface onto which the insulator layer is created may beslightly oxidised prior to coating. Suitable metals are copper, iron,molybdenum, nickel, platinum, tantalum, titanium, tungsten. Suitablealloys are steels, nickel-iron, chromium-iron, nickel-chromium-iron,nickelcobalt-iron. The oxidation may be controlled by a careful choiceof atmosphere e.g. wet hydrogen in the same manner as glass to metalsealing. The oxide formed may be an insulator or it may react with theinsulator layer to form a layer of intermediate properties, graded fromconductive adjacent the metal surface to insulating adjacent theinsulator layer. Such a a layer of intermediate properties facilitatesthe tunnelling of electrons from the substrate into the insulator.

Let us now move on to the uses of these teachings in practical emitters.It should be understood that the following descriptions are onlyillustrative of certain embodiments of the invention. Variousalternatives and modifications can devised by those skilled in the art.

FIGS. 3a to 3 d show some uses of optimised insulating coatings inemitter systems. In all cases the conducting substrate is labelled 1700and the conducting channel and its associated electron emission 1701.The optimised interface layer between the substrate 1700 and theinsulator 1703 is labelled 1702, and can be created in any of the wayspreviously described. FIGS. 3a and 3 b show conducting particle basedMIV emitters as previously described in our patent application GB 2 332089. FIG. 3c is a MIMIV emitter as described by Tuck, Taylor and Latham(GB 2 304 989). FIG. 3d is a microfabricated tip emitter. The basicprinciples of the emission of electrons will be apparent from theforegoing description, and are therefore not repeated again here.

FIGS. 4a to 4 d show how an optimised surface region 1800 of theinsulator coating 1703 may be used in the same emitter systems aspreviously described and detailed in FIGS. 3a to 3 d. FIG. 4acorresponds with FIG. 3a et cetera as do the reference numbers anddescriptions. The optimised surface region 1800 can be created in any ofthe ways previously described. The basic principles of the emission ofelectrons will be apparent from the foregoing description, and aretherefore not repeated again here.

Preferred embodiments of the invention provide emitting materials whichare designed deliberately to have a significant density of emittingsites, as opposed to accidental and unwanted sparse inclusions ofsporadic emitters, as have been noted from time to time in the vacuuminsulating field, for example.

In preferred embodiments of the invention, the distribution of emittingsites over the field electron emission material is preferably random,with an average density of at least 10² cm⁻², 10³ cm⁻², 10⁴ cm⁻² or 10⁵cm⁻². The distribution is also substantially uniform and, preferably,when using a circular measurement area of 1 mm in diameter, issubstantially Binomial or Poisson. The uniformity may be such that thedensity of the emitting sites in any circular area of 1 mm diameter doesnot vary by more than 20% from the average density of distribution ofsites for all of the field electron emission material. The distributionof the emitting sites over the field electron emission material may havea uniformity such that there is at least a 50% probability of at leastone emitting site being located in any circular area of 4 μm or 10 μmdiameter.

In this specification, the verb “comprise” has its normal dictionarymeaning, to denote non-exclusive inclusion. That is, use of the word“comprise” (or any of its derivatives) to include one feature or more,does not exclude the possibility of also including further features.

The reader's attention is directed to all papers and documents which arefiled concurrently with or previous to this specification in connectionwith this application and which are open to public inspection with thisspecification, and the contents of all such papers and documents areincorporated herein by reference.

All of the features disclosed in this specification (including anyaccompanying claims, abstract and drawings), and/or all of the steps ofany method or process so disclosed, may be combined in any combination,except combinations where at least some of such features and/or stepsare mutually exclusive.

Each feature disclosed in this specification (including any accompanyingclaims, abstract and drawings), may be replaced by alternative featuresserving the same, equivalent or similar purpose, unless expressly statedotherwise. Thus, unless expressly stated otherwise, each featuredisclosed is one example only of a generic series of equivalent orsimilar features.

The invention is not restricted to the details of the foregoingembodiment(s). The invention extends to any novel one, or any novelcombination, of the features disclosed in this specification (includingany accompanying claims, abstract and drawings), or to any novel one, orany novel combination, of the steps of any method or process sodisclosed.

What is claimed is:
 1. A method of creating a field electron emissionmaterial, comprising the steps of: providing a substrate having anelectrically conductive surface; providing a plurality of electronemission sites on said conductive surface, each of said sites includinga respective layer of electrically insulating material to define aprimary interface region between said conductive surface, or anelectrically conductive particle on said conductive surface, and saidinsulating layer, and a secondary interface region between saidinsulating layer and the environment in which the field electronemission material is disposed; and treating or creating the primaryinterface region of each said layer so as to enhance the probability ofelectron injection from said conductive surface into said layer, suchtreatment or creation comprising: depositing a layer of material betweensaid conductive surface and insulating layer, which layer of materialhas properties intermediate those of said conductive surface and saidinsulating layer; or doping said conductive surface and/or insulatinglayer with a material that segregates out at said primary interfaceregion during subsequent processing; or reaction of the materials ofsaid conductive surface and insulating layer; or creating said primaryinterface region as a region of high electrically active doping, highdefect density or intermediate chemical composition: such that saidprimary interface region after said treatment or creation is either aninsulator or graded from conducting adjacent said conductive surface toinsulating adjacent said insulating layer.
 2. A method according toclaim 1, wherein said layer of material between said conductive surfaceand insulating layer is created by a gradual change in stoichiometry,composition or doping of the material of the layer, to reducediscontinuity.
 3. A method according to claim 1, further comprising thestep of selecting the properties of said insulating layer of each saidsite between the respective said primary and secondary interface regionsto limit the emission current flowing through said layer to apredetermined value.
 4. A method according to claim 1, wherein saidsubstrate is of metal and said primary interface region is a layer ofmaterial of low work function.
 5. A method according to claim 1, whereinsaid primary interface region is created as a region of high doping,high defect density or intermediate composition.
 6. A method accordingto claim 5, wherein a region of high defect density is created by heattreating a major portion of a highly defective insulator material tocreate said insulating layer, whilst avoiding heat treatment of an endportion of said highly defective insulator material, which end portionthen remains as said region of high defect density.
 7. A methodaccording to claim 1, wherein said secondary interface region isprovided by modifying the surface of said insulating layer, to enhancethe probability of electron transmission from said insulating layer tosaid environment.
 8. A method according to claim 7, wherein modificationof said surface is by a local increase in defect density of the materialof the insulating layer.
 9. A method according to claim 7, whereinmodification of said surface is by a gradual change in stoichiometry,composition or doping to reduce discontinuity.
 10. A method according toclaim 1, wherein some or all of said electron emission sites are definedby tips or projections created on said electrically conductive surfaceof said substrate.
 11. A method according to claim 1, wherein some orall of said electron emission sites are defined by electricallyconductive particles coated on said electrically conductive surface ofsaid substrate.
 12. A method according to claim 1, wherein saidsecondary interface region is defined at a region of said insulatinglayer between an electrically conductive particle and said electricallyconductive surface of said substrate.
 13. A method according to claim 1,wherein said secondary interface region is defined at a region of saidinsulating layer which is provided on a portion of a respective saidparticle which faces away from said conductive surface.
 14. A methodaccording to claim 1, wherein each said particle has a first layer ofelectrically insulating material between said substrate and particle anda second layer of electrically insulating material between said particleand environment, the arrangement being such that, in use, electronemission takes place by injection of electrons through one said primaryinterface region defined between said substrate and said firstinsulating layer, by injection of electrons through another said primaryinterface region defined between said particle and said secondinsulating layer, and by transmission of electrons through saidsecondary interface region defined between said second insulating layerand said environment.
 15. A method according to claim 14, wherein saidfirst and second insulating layers are provided by respective portionsof a common electrically insulating material.
 16. A method according toclaim 1, wherein said insulating layer is of a material other thandiamond.
 17. A method according to claim 1, wherein the distribution ofsaid sites over the field electron emission material is random.
 18. Amethod according to claim 1, wherein said sites are distributed over thefield electron emission material at an average density of at least 10²cm⁻².
 19. A method according to claim 1, wherein said sites aredistributed over the field electron emission material at an averagedensity of at least 10³ cm⁻², 10⁴ cm⁻² or 10⁵ cm⁻².
 20. A methodaccording to claim 1, wherein the distribution of said sites over thefield electron emission material is substantially uniform.
 21. A methodaccording to claim 20, wherein the distribution of said sites over thefield electron emission material has a uniformity such that the densityof said sites in any circular area of 1 mm diameter does not vary bymore than 20% from the average density of distribution of sites for allof the field electron emission material.
 22. A method according to claim20, wherein the distribution of said sites over the field electronemission material when using a circular measurement area of 1 mm indiameter is substantially Binomial or Poisson.
 23. A method according toclaim 20, wherein the distribution of said sites over the field electronemission material has a uniformity such that there is at least a 50%probability of at least one emitting site being located in any circulararea of 4 μm diameter.
 24. A method according to claim 20, wherein thedistribution of said sites over the field electron emission material hasa uniformity such that there is at least a 50% probability of at leastone emitting site being located in any circular area of 10 μm diameter.25. A field electron emission device comprising a field electronemission material produced by a method according to claim 1, and meansfor subjecting said material to an electric field in order to cause saidmaterial to emit electrons.